The present invention concerns electron tubes with at least one grid, used as amplifiers, which produce signals with a great variation in amplitude. Its object is to improve their efficiency.
It concerns more particularly tubes for television emitters using negative transmission. Most countries in Europe and even throughout the world use negative transmission. France, Luxembourg and some other countries use positive transmission.
The tubes used in television emitters are generally tetrodes. The invention can of course apply to other grid tubes such as triodes, pentodes, etc.
The amplifier tubes used as television emitters generally operate as class B amplifiers. The invention can also apply to tubes operating as class AB, B or C amplifiers.
A triode is a tube with a cathode emitting electrons, an anode or plate and a single grid, known as a control grid, placed between the cathode and the anode.
A tetrode has an additional grid known as a screen grid, placed between the control grid and the anode.
A pentode has one grid more than the tetrode, this grid known as a suppressor grid is and is inserted between the screen grid and the anode.
A television transmitter emits a video signal comprising notably the repetition of the following two signals: a synchronization signal and a video line signal. The period of repetition of these two signals is about 64 microseconds in 625-line systems.
The synchronization signal is an approximately rectangular pulse with a duration of about 4.6 microseconds. This duration is very short compared with the repetition period. The line video signal lasts for the rest of the repetition, i.e. about 59.4 microseconds.
In a negative transmission system the transmitter's maximum power is delivered during the synchronization signal. The maximum level of the video line signal corresponds to black, and the minimum level of the video line signal corresponds to white.
In a positive transmission system, on the other hand, the synchronization signal corresponds to the minimum power of the transmitter. The maximum level of the video line signal corresponds to white and the minimum level to black.
Hereinafter we shall refer only to negative transmission systems.
In order to obtain a sufficiently powerful synchronization signal at the output from the transmitter, the anode of the tube must receive a high polarization voltage which is much higher than the voltage necessary to obtain the power corresponding to black.
This polarization voltage serves only during the synchronization signal and is unnecessarily high for the whole duration of the video line signal.
The tube's efficiency is proportional to the ratio of the voltage of the video signal emitted by the tube to the polarization voltage of the anode. As the duration of emission of the line signal is much longer than the duration of emission of the synchronization signal, the overall efficiency of the tube is practically that of the video line signal. And this efficiency is very poor owing to the unnecessarily high polarization voltage.